1. Field of the Invention
This invention relates to an ultrasound internal examination system employing an ultrasound vibratory element to be inserted, for example, into a human body by way of an endoscope or the like for scanning a particular range of concern for inspection or for diagnostic purposes.
2. Description of the Prior Art
The ultrasound examination systems which have been widely used in medical fields generally includes an ultrasound vibratory element and an ultrasound image observation terminal which is largely constituted by ultrasound signal transmission and reception circuits, a signal processor including circuits for processing ultrasound echo signals received by the reception circuit, and a monitor for displaying an ultrasound image on the basis of the output signal of the signal processor.
It has been known in the art to insert an ultrasound vibratory element into a patient's body to a position close to a target portion of examination or diagnosis for the purpose of improving the accuracy and functions of the ultrasound examination and diagnosis. For instance, U.S. Pat. No. 4,802,487 discloses an ultrasound probe which has a single-element ultrasound transducer at the tip end of a narrow flexible cable. Through a biopsy channel which is provided in an endoscope for insertion of forceps or other instruments, the ultrasound probe is inserted into a patient's body, protruding the ultrasound transducer at the tip end of the probe from the fore end of an insert portion of the endoscope to contact an intracavitary wall such as an alimentary tract wall or the like or to confront an intracavitary wall through an ultrasound transmissive medium thereby to transmit ultrasound pulses into the body while receiving return echo signals. The ultrasound probe is scanned in a linear or radial direction manually or by a drive means like a motor to obtain a B-mode ultrasound image.
This ultrasound probe arrangement makes it possible to obtain a cross-sectional image of intracavitary wall tissues of interest in addition to endoscopic examination and diagnosis, permitting three-dimensional examination for diagnosis of high accuracy.
In this connection, the ultrasound transducer element needs to be scanned to get a B-mode ultrasound image. However, for this purpose, the single-element ultrasound transducer has to be moved mechanically by pulling the flexible cable back and forth in the linear direction or by turning the transducer in the radial direction. At this time, it is necessary to generate an ultrasound pulse in a predetermined pitch and, for determining the display position of each acoustic line of the ultrasound image to be shown on the monitor on the basis of the ultrasound return echo signal, to obtain the information indicative of the position of the ultrasound transducer in scanning. In this regard, the above-mentioned prior art employs a potentiometer in a position sensor mechanism for detecting the position of the ultrasound transducer element. For example, in linear scanning, a contact member is provided at the proximal end of the ultrasound probe in contact with a resistor of the potentiometer, sliding the contact member along the resistor as the ultrasound probe is manipulated back and forth and detecting the position of the ultrasound transducer in the scanning stroke by way of a voltage signal proportionate to the variation in resistance.
The above-mentioned potentiometer, which produces a voltage or analog signal as a measure of the transducer position, has an inherent drawback that it is apt to be influenced by voltage fluctuations in the power supply and external noises. Besides, when detecting the transducer position on the basis of the voltage signal from the potentiometer, the output signal is compared by a comparator to determine a difference in voltage level. Therefore, in order to enhance the resolution of distance, such voltage level differences of comparison have to be minimized. For example, in a case where the full scanning stroke range is 20 mm and ultrasound signal of about 500 pulses is transmitted in that range, the pulse intervals become extremely narrow, such as less than 40 .mu.m. Since the position signal from the potentiometer is an analog signal, theoretically its resolution of distance is infinite but, considering the signal stability and reliability, it is practically incapable of coping with the high distance resolution as mentioned above.
With a view to overcoming this problem, the inventors studied the possibility of replacing the potentiometer by an encoder as a position sensor for the ultrasound transducer or vibratory element. Encoders can be largely classified into: an absolute type which can detect an absolute position by means of a rotary disc having a detection pattern with a multitude of detection units in a predetermined pitch at predetermined angular intervals, in association with an optical or other sensor means adapted to produce pulse signals according to the number of detection units read in; and an increment type which can detect the occurrence and extent of a displacement but not the absolute position. The absolute encoder which is capable of detecting the absolute position can replace the potentiometer to a sufficient degree in terms of functions. However, in order to secure an extremely high resolution of distance, for example, a distance resolution of 40 .mu.m or less, there has to be provided an encoder an objectionably large diameter along with an increased number of sensors in association with the detection pattern on the encoder disc. In addition to high resolution of distance, there has been a strong demand for the ultrasound transducer position sensor, which is usually mounted on a manipulating portion of an endoscope, to be compact and small in size and weight from the standpoint of the operationability of the endoscope. Accordingly, it is practically impossible to employ a large absolute encoder having high distance resolution of 40 .mu.m or less. On the other hand, the incremental encoder which can detect displacement of the ultrasound transducer is not capable of detecting its position and thus cannot be applied to the transducer position sensor as it is.